This invention relates to methods and apparatus for coating a product, particularly but not exclusively a food product, in a cooling chamber. In particular, the invention relates to the controlled application of a coating material to a food product within a tumble cooling apparatus.
Tumble coolers, or tumblers, are widely used and well known in the art of processing food products of the type comprising a plurality of discrete pieces or portions, such as raw meat pieces (e.g. ham or chicken nuggets), or fish (e.g. prawns, scampi, pieces of cod). The product is agitated by being "churned" in a drum which rotates about a substantially horizontal axis, whilst being sprayed with a coolant, either a liquid cryogen, such as nitrogen, or a gas at low temperature, such as carbon dioxide. The liquid cryogen mixes thoroughly with the particulate product, aided by the churning, or tumbling, effect of the rotating drum, and produces a marked reduction in the temperature of the product, to chill, freeze or "deep freeze" the product as required. Tumblers may be used to chill in batches, food being loaded in batches into the tumbler, chilled and then discharged from the tumbler, or they may be arranged as continuous cooling devices in which the food is fed into one end of the cylindrical drum, travels along the drum in an axial direction whilst being chilled and then is discharged from the other end of the drum. These two types of tumble coolers will hereafter be referred to as "batch tumblers" and "continuous tumblers", respectively (the present invention being applicable to both).
Such tumblers are now beginning to be used to apply coating materials to food products, for example coating materials such as flavored oils, sauces, batters and so on. This is accomplished by cooling the food product to a temperature substantially below the freezing point of the coating material and then spraying the coating material into the still-rotating tumbler and onto the churning food product so that the coating material freezes and adheres to the food product. Again, the continuous agitation of the food product helps to ensure that an even coating is applied, and also to prevent agglomeration of the product. A typical application would be to cool vegetable pieces to about -50.degree. C. for a sauce with a freezing point of -3.degree. C.; the sauce is then applied to the vegetables during tumbling (but usually whilst spraying with liquid cryogen has ceased) until a fairly even coating is produced.
When using a tumbler for coating a product, the cooling of the product has to be to the correct temperature, uniform and homogeneous, so as to ensure that each product piece is coated evenly and that all product pieces are coated to the same extent. If the product is too warm, the coating material does not freeze quickly enough to adhere firmly to the product. If the product is too cold, the coating material tends to freeze too fast, so that droplets of coating material larger than are desirable for an even coating accumulate on the surface of the product. If the product is much too cold, the coating material can freeze to, but subsequently de-bond from, the product due to the thermal contraction of the coating. If the core is very much too cold, the coating and/or the product may become brittle and break up due to the mechanical stresses produced by the churning of the tumbler, or simply as a result of the thermal stresses produced in the coating and/or the product.
The conventional methods used to control cooling in processes in which the tumbler is used for chilling/freezing only, such as timed application of liquid cryogen, controlled spray pressure and temperature feedback (i.e., adjusting liquid cryogen input in accordance with the temperature of the cryogen vapour leaving the tumbler) are unsuitable for tumble coating applications, since they are too coarse, and cannot produce a consistently uniform and properly adherent coating.
It is an aim of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, and hopefully to provide an effective method of tumble coating which is something of an improvement over the prior art.